1. Field of the Invention
This invention relates to field-flow fractionation, a technique used for separation and characterization of complex polymeric, macromolecular and particulate materials. More particularly, the invention relates to a new technique for programming both field and flow in field-flow fractionation.
Specifically, the invention provides a new technique for programming of field decay or of increase of channel flow in field-flow fractionation, which I call power programming, which imparts improved uniformity of fractionating power. The new process comprises an improvement in programming a field-flow fractionation wherein a carrier fluid containing particles or macromolecules to be separated is forced through a thin flow-channel and a field or gradient is used to induce a driving force acting across the thin dimension perpendicular to the fluid axis, said improvement involves varying one of the parameters that affect the interaction of the sample components with the field and fluid medium in order to reduce separation time and better equalize component separation, said parameters including decreasing the field strength according to the power equation set out below, or increasing the flow rate according to the other equation set out below, or use of both parameters together.
The improvement involving the variation of the field strength comprises holding the field strength S constant at an initial level S.sub.0 for a time-lag period of t.sub.1, and then decaying the field strength with time t according to the equation ##EQU1## with the requirements that t.gtoreq.t.sub.1 &gt;t.sub.a, t.sub.1 .gtoreq.0 and p&gt;0, and where S is the field strength at time t, S.sub.0 is the initial field strength, and t.sub.1 is a time lag following the start of elution through the field-flow fractionation (FFF) system during which the field is held constant at S.sub.0. The remaining time parameter t.sub.a and the power p take particular optimum values when highly polydisperse materials are to be characterized, generally with t.sub.a =-p t.sub.1 and t.sub.1 .noteq.0.
The improvement involving the variation of the mean flow velocity utilizes a program taking the form. ##EQU2## wherein t.gtoreq.t.sub.2 &gt;t.sub.b, t.sub.2 .gtoreq.0 and q&lt;0 for a programmed increase in flow velocity, and where &lt;v&gt; is the mean channel flow velocity at time t, &lt;v&gt;.sub.0 is the initial mean flow velocity and t.sub.2 is a time lag during which &lt;v&gt; is held constant at &lt;v&gt;.sub.0. The remaining time parameter t.sub.b and the power q will take, as with the power programmed field operation, values consistent with lessened variation of fractionating power over wide particle size or molecular weight range, generally with t.sub.b =q t.sub.2 and t.sub.2 .noteq.0.
The invention also provides a process involving the power programming of both the field and flow velocity simultaneously to provide advantages not obtainable when programming either the field or flow alone.
2. Prior Art
There is a growing need in industry and health sciences for the separation of submicron particles including latices, environmental particles, carbon black, industrial powders, crystallization products, and related particulate matter.
Various methods have been proposed, but in general, they have been slow, too low in throughput, inefficient, expensive or have failed to effect the separation with the desired degree of resolution needed for commercial operations.
Some of the highest resolution techniques disclosed have been those based on field-flow fractionation as disclosed in U.S. Pat. No. 3,449,938 and U.S. Pat. No. 4,147,621, but their resolution leaves much to be desired. For example, the technique is often impractical because of the inordinate time required for elution of the larger particles. For example, when the technique is used to fractionate wide-ranging mixtures problems are encountered due to the incomplete resolution of early peaks and the excessive retention time and peak width of late peaks.
The solution to the above problem is often effectively realized in various programming techniques. For example, various retention-influencing parameters, such as temperature, solvent properties, field strength, flow velocity, etc. are varied in the course of a run in order to expose in an orderly sequence each of the components to effective separation conditions.
Field programming has been most extensively developed. In this technique the field strength is reduced with time, gradually according to some specific mathematical function. Parabolic field decay (linear rpm decay) and parabolic decay combined with an abrupt reduction of field strength were used in the earliest programmed sedimentation FFF experiments (Yang et al., Anal. Chem. 1974 46, 1924). Linear and parabolic field decay combined with an initial time lag period were utilized for thermal FFF (Giddings et al., Anal. Chem. 1976 48, 1587). Subsequently, Kirkland, Yau, and co-workers introduced a widely applied exponential field decay with and without time lag (Kirkland et al., Anal. Chem. 1980 52, 1944).
These proposed programming techniques, however, have various limitations which have restricted their ability to fractionate particulate, polymeric and macromolecular samples uniformly. For example, with the exponential decay program there is a great variation of fractionating power with either particle diameter or molecular weight. The limitations of these prior known techniques are illustrated in the comparative data presented hereinafter.
It is an object of the invention, therefore, to provide a new technique for the programming of field strength and channel flow in field-flow fractionation. It is a further object to provide a programming technique which gives improved uniformity of fractionating power over a substantial particle diameter or molecular weight range. It is a further object to provide a new mathematical form for field decay which, with the proper adjustment of parameters, yields uniform plots of fractionating power vs particle diameter or vs molecular weight. It is a further object to provide a programming technique which gives shorter analysis time at some required minimum fractionating power with a broader range of particle size or molecular weight than is obtainable with previously proposed techniques. It is a further object to provide a new programming technique for FFF which involves programming of field strength, channel flow velocity as well as a combination of both. These and other objects of the invention will be apparent from the following detailed description thereof.